fucking hard. The kind of guy you wouldn't mess with under any condition, like Gary Busey in Lethal Weapon. Normally quite squat, very powerfully built. People who are Nails shirk off car/plane crashes as if they have grazed their knee. Crazy russians who wrestle bears are nails.

faaaaking ell.. large Larry took aht 20 cozzers before they got the cuffs on im... then he chewed off 'is own aaand to escape. E's fucking nails I tell yah.

Nothing in particular, except the person is usually known to cause pain or to be able to handle what others would call intense amounts of it.

They often drink beer, smoke cigarrettes, and go out with sexy girls.

True story.

My neighbor was nails, he was in a car accident where his truck rolled many times without a seatbelt. He got out to smoke a cigarrette, and went and stood by the drunk man that hit him, communicating, with the man missing bodily parts and expelling massive quantities of fluid.